1. Field of the Invention
The present invention relates generally to apparatus for collecting precipitation from a solvent solution and in particular to apparatus for recovering silver from spent photographic processing solutions.
2. Description of the Prior Art
Silver recovery from spent photographic processing solutions represents a significant means for conserving silver as a natural resource. An equally important consideration is that heavy metals such as silver and other compounds typically found in spent photographic solutions should not be discharged into municipal storm sewers or sewage systems because of the general toxic effect on the ecology and interference with operation of the sewage system and because of the danger of contaminating the local aquifer. Furthermore, because silver is an important industrial metal, and because it is relatively scarce, the recovery of silver from spent photographic processing solutions represents significant source of revenue for industrial users of photographic products.
Because of the intrinsic value of silver, there has been considerable interest down through the years in providing apparatus for recovering the silver values from photographic solutions. Such apparatus typically includes a metal deposit in the solution flow path which is higher than silver in the electromotive series, and the silver is deposited on this metal as the metal replaces silver in the solution as a result of a chemical reduction reaction. The prior art teaches the use of steel or copper wool, metal turnings, wire, chips and the like interposed in the flow path of the solution. The complex silver salts formed during film processing yield the silver on contact with the metal which is above silver in the electromotive series, thereby causing that metal to go into solution in place of the silver ions, which become deposited on the remaining metal as zero valence silver.
Such conventional silver recovery units have been generally satisfactory as far as the economy of relatively low volume operations is concerned. However, elaborate multi-stage arrangements of prior art recovery devices have been required to remove enough silver from the photographic solutions to meet the various standards established by governmental agencies concerning the maximum amount of a heavy metal such as silver, in parts per million, which is permissible in effluent discharge into a water course or sewage system. Additionally, in large scale recovery operations, the flow rate of photographic solution through recovery devices is a limiting factor in regeneration or rejuvenation processes as well as being a limiting factor in the silver recovery process. Therefore there is a continuing interest in improving the basic silver recovery procedure to make it more effective and efficient.